Homologous amino acid sequences in choleragen and glycopeptide hormones such as thyrotropin have led to speculation that choleragen and glycopeptide hormones activate adenylate cyclase by similar mechanisms. We have studied adenylate cyclase of a particulate preparation of bovine thyroid. Nicotinamide adenine dinucleotide (NAD) is required for activation of the enzyme by choleragen, but not by thyrotropin. Thyrotropin bound to thyroid particles and subsequently eluted retained the capacity to activate adenylate cyclase, whereas exposure to thyroid particles greatly diminished the capacity of choleragen to activate adenylate cyclase.